DLG4 and major depressive disorder: The levels of various proteins that serve as markers of synaptic plasticity, such as microtubule-associated protein 2 (MAP-2), postsynaptic density 95 (PSD-95), and synaptophysin (SYN), are decreased in the hippocampus [17], and the severity of depression is significantly related to the number of hippocampal spine synapses [18, 19].